Nearly a year after the first cases of Zika were diagnosed in Brazil, the virus, which is suspected to cause birth defects and other neurological problems, is bearing down on American shores. It is already in Puerto Rico and the United States Virgin Islands. There have been more than 50 cases of Americans infected abroad, and most experts believe that by summer, the continental United States will have some of its own homegrown cases, meaning that domestic mosquitoes will have the virus.

“It’s like preparing for a hurricane,” said Dennis Moore, director of mosquito control in Pasco County, Fla. “You know it’s coming from across the Gulf, but where it’s going to land, we just don’t know.”

How Mosquitoes Spread Zika

The Aedes aegypti mosquito is thought to be responsible for most of the spread of Zika. The virus is carried by female mosquitoes (males do not bite) that have fed on infected blood.

Scientists say an explosive spreading of Zika is extremely unlikely in the continental United States. But Aedes aegypti, the mosquito that carries the virus, is tenacious and relatively impervious to broad outdoor spraying. Techniques for tracking it are outdated and underfunded. Experts interviewed here this week said fighting Zika will require a major shift in this country’s approach to mosquito control, namely more door-to-door action, a painstaking and expensive practice that many say is a tall order in an era of shrinking budgets and wariness of government intrusion.

“This guerrilla warfare house-to-house method is still very new and I’m not convinced that many places are prepared for it,” said Michael Doyle, the head of mosquito control for the Florida Keys, a district that used such tactics during an outbreak of the dengue virus in 2009 and 2010, one of the first major outbreaks in the United States since the 1930s.

The traditional technique of spraying from trucks and planes “is like fighting Al Qaeda with an old-fashioned army,” he said.

The United States’ best defense against Zika is its air-conditioning and window screens, scientists said. Yellow fever and malaria killed tens of thousands of Americans through the end of the 19th century, but higher living standards helped stop it. Screens and round-the-clock air conditioning block virus transmission by keeping healthy mosquitoes from biting infected people and spreading the virus. They also prevent infected mosquitoes from biting healthy people.

“You won’t see anything like what you see in the tropical countries,” said Scott Ritchie, a medical entomologist at the Australian Institute of Tropical Health and Medicine, sitting among booths here displaying insecticide blowers and mosquito bait that uses date juice. (A mosquito wine stopper was given away in a conference raffle.) “Americans have hermetically sealed, air-conditioned houses. Explosive transmission with aegypti only takes place in these tropical areas that have a lot of unscreened housing.”

Even so, businesses in the United States have been rattled. And some experts are worried that some poor neighborhoods, where air-conditioning is less ubiquitous, may be more vulnerable.

“There’s big concern,” said Dan Ariaz, who was displaying wares from his Las Vegas company, Arro-Gun Spray Systems, in the back of a 1934 Chevrolet pickup. His phone has been ringing off the hook, including calls from resorts needing advice. “We tell them not to panic,” said Mr. Ariaz, whose shirt collar was embroidered with the words, “Bite Me.”

At first glance, Aedes aegypti may not seem like a formidable foe. It hastraditionally stayed in a corner of the American South, rarely venturing farther north than South Carolina. It does not fly farther than a city block, and usually lives less than two weeks.

“It’s the cockroach of mosquitoes,” said Dr. Thomas R. Frieden, the director of the Centers for Disease Control and Prevention. “It lives indoors around people and hides in dark places.”

That makes it much harder for droplets sprayed from airplanes or trucks to come into contact with it to kill it. (Floodwater mosquitoes, the ones that bother Americans on summer nights, rarely spread disease. Spraying works on them because they are often flying.)

“There’s going to be a lot of pressure for districts to go out and spray, but that is not very likely to work against these mosquitoes,” said Joseph Conlon, a retired Navy entomologist who is a technical adviser to the American Mosquito Control Association.

One key test of Americans’ ability to fight this mosquito came in 2009, when it brought the dengue virus into the homes of people on Key West. Within hours, a SWAT team of 30 mosquito experts was going house to house, dumping standing water from flowerpots, ashtrays, children’s swimming pools, recycling containers, bottle caps and trash cans. They worked 10 hours a day, six days a week, for six weeks, marking their progress on maps.

The effort was scrupulous because “you need remarkably few mosquitoes to keep up transmission,” said Mr. Doyle. “One recalcitrant neighbor will grow enough adult mosquitoes to cause an outbreak for the whole neighborhood.”

In all, Key West had at least 85 cases of dengue in 2009 and 2010. The virus never spread beyond the island and the outbreak eventually died out. The district spends $1 million a year just on fighting the Aedes aegypti mosquito, about a tenth of its mosquito control budget and as much as some counties spend in total. Miami-Dade, with a far larger population, has a budget of just $1.8 million.

Some departments get creative. The mosquito control team in New Orleans got firefighters to help knock on doors when cases of West Nile virus began surfacing in 2012.

“Manpower, that’s the main thing,” said Claudia Riegel, director of the city’s Mosquito, Termite and Rodent Control Board.

Tracking the mosquito is critical, but the country’s ability to do that is spotty. Mosquito control departments, often called districts, range from “the enormously sophisticated to the sanitation guy spraying a few places when he thinks about it,” Dr. Frieden said. Mr. Conlon estimates there are about 700 across the country with the ability to track mosquitoes and another 1,100 smaller ones that probably cannot afford to.

Mosquito control for Chatham County, which includes Savannah, traps extensively, with records going back years. But Jeffrey Heusel, its director, said it is unusual: Of about 60 counties in Georgia that once had some ability to trap, now only about six do. According to the C.D.C., epidemiology and laboratory funding for so-called vector-borne diseases (those spread by mosquitoes, ticks, fleas and other creatures) dropped to just $9.5 million in 2015 from about $24 million in 2004.

The aegypti maps that the C.D.C. uses are outdated and part of the $1.8 billion that President Obama requested for Zika this week would go to improving them, Dr. Frieden said.

“The first thing you have to do is monitoring so we know where they are,” he said.

The Zika-carrying mosquito will come out in full in the United States in summer. Dr. Frieden said the C.D.C. planned to put together teams of experts to help states handle clusters of cases. The agency has shipped an ample national supply of material the states can use to test a sick person’s blood for Zika. The more complex test, to detect whether someone was sick months ago, is taking more time, and sufficient supply may take a few more weeks or even months, he said. Still, he was hopeful that Congress would act quickly.

“Are we ready? That’s the question of the hour,” Dan Strickman, a medical entomologist at the Bill & Melinda Gates Foundation, told an eager crowd at the conference. “When we apply ourselves to a particular mosquito problem, we can overcome it.”

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